Fabrication of novel micro–nano carbonous composites based on self-made hollow activated carbon fibers
Graphical abstract
Highlights
▸ Hollow pipe and porous HACF with solid carbon net framework structure were successfully prepared by template method. ▸ CNTs were grown successfully on the self-made HACF substrate by CVD techniques. ▸ A novel tree-like micro-nano carbonous structure CNTs/HACF was fabricated. ▸ The formation mechanism of micro phase HACF and nano phase CNTs were respectively discussed.
Introduction
Activated carbon fibers (ACF) are characterized by high carbon contents, large specific surface areas, abundant micropores and small or narrow aperture. Hollow activated carbon fibers (HACF) are a kind of specific ACF with hollow structures. There are enrich micropores on both the internal and external surfaces of HACF, hence make it has larger specific surface areas, and possess unique advantages of fast adsorption rate, large adsorption capacity and simpler regeneration in the gas–liquid adsorption fields [1], [2], [3].
For a long time, most hollow activated carbon fibers (HACF) were prepared based on spinning technologies [2], [3], [4], [5], [6], which usually use various kinds of organic polymer fibers including polyacrylonitrile fiber, cellulose fiber, polyethylene fiber, and polyvinyl alcohol fiber, etc. to get hollow fiber strands, and then repass preoxidation, carbonization process to obtain expected carbon fibers with hollow structures. However, the spinning process is very complex, including the proper selection of polymer precursors, the parameters involved in electrospining process, the pre-oxidation, carbonization and activation techniques, etc. In addition, it is difficult to control the pore size distribution and pore connectivity. In recent years, alternative controllable and cost – effective HACF synthetic routes – template method has aroused intensive attention, by using the polymer or organic fibers (polypropylene hollow fiber, polyvinyl alcohol fiber, organic silica fiber, etc.) as template, carbon-rich compounds (sucrose, polystyrene, etc.) as carbon precursors. After template materials were decomposed or removed at certain temperatures, hollow structured carbon fiber with ideal regular and uniform morphologies can be obtained. Shi et al. [7] firstly reported the synthesis of hollow carbon fibers (HCFs) by using polypropylene hollow fibers templates with sucrose as carbon precursors. Cheng et al. [8] prepared circular and rectangular opening HCFs by using electrospun silica fibers as template and polystyrene as carbon source. Fatema et al. [9] reported the HCFs fabrication using PVA fibers with the aid of iodine pretreatment.
The discovery of carbon nanotubes (CNTs) by Iijima [10] has attracted great attention because of their unique structural, thermal, adsorptive, electronic, and mechanical properties, etc. [11], [12], [13]. Chemical vapor deposition (CVD) is a simple and economic technique for synthesizing CNTs at low temperature and ambient pressure with various substrates [14]. There have been many reports about CNTs growth on different carbon matrices, such as carbon fibers [15], [16], [17], carbon paper (CP) [18], [19], active carbon [20], [21], and ordered mesoporous carbon [22]. However, there is still no report about CNTs deposited on the hollow activated carbon fibers so far to the best of our knowledge.
In the present work, HACF has been successfully prepared by template method using polypropylene hollow fibers as template, and phenol-formaldehyde resin is firstly used as carbon precursor. What more important, novel micro–nano CNTs/HACF composites are obtained using HACF as the substrate through CVD technology.
Section snippets
Preparation process of the micro–nano carbonous composites CNTs/HACF
- (i)
Preparation of hollow activated carbon fibers (HACF)
A polypropylene hollow fiber (PPHF) (400 μm I.D., 30–40 μm wall thickness, 0.2 μm pore size, from Hangzhou Kaihong Membrane Technology Co., Ltd., China) was used as the template. Firstly, the pristine PPHF were cut into short fibers with about 3 cm long, and purified with alcohol for 24 h. Subsequently, purified PPHF was immersed into phenol-formaldehyde resin (PF) solution (PPHF:PF:ethanol = 0.25 g:5 g:25 mL) for 24 h, and gradually dried at 50 °C, 80 °C,
Morphologies and structures of the HACF and CNTs/HACF
The morphologies, surface structures, and dimension of HACF and CNTs/HACF hybrids were characterized by SEM at different magnifications and view angles. Fig. 1(a) and (b) displays the cross-sectional images of the obtained HACF at different magnifications. The hollow structure indicates that thermal decomposition of PPHF does not result in the collapse of the fibers. The external and internal diameters of the HACF are about 420 and 330 μm, respectively, and the wall thickness of HCF is about 90
Conclusions
In summary, hollow pipe and porous HACF with solid carbon net framework structure was successfully prepared by template method using phenolic resin as carbon precursor, which provided convenient for the loading of metal catalyst due to its porous substrate characteristics. Furthermore, the as-grown HACF was firstly used as substrate to grow CNTs through CVD technology. After a series of synthetic process, CNTs with diameters of 70–90 nm are acquired, and most of them seems to grown perpendicular
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